First make sure the tapes are HDV and not shot DV. Secondly check menus in the camera to make sure it is exporting as HDV and not downconverting to DV via the firewire connector.
Next make sure FCP has the correct East Setup for HDV at the right frame rate. If all is set correctly, put a tape in the camera. Play a bit of the tape so the camera recognises the correct format. Open up Log & Capture and make sure you have machine control. At this point you should be able to control the camera as a deck and see images from tape. Sometimes you have to go through a specific sequence of powering computer, connecting camera, powering camera, loading tape, opening FCP software and the L&C.
Finally if no luck then it might mean you need to trash preferences. I have seen FCP ignore firewire inputs until prefs were trashed. Use Digital Rebellion’s free Prefs Manager. There is a case where HDV shot on a Canon in Progressive mode will only playback with a canon camera. Sometime the Canons need the firewire basic setup to work in the Easy Setups.
Finally never fear a search of this forum as HDV capture problems have been discussed here often.